# A world inhabitable for half the time [closed]

So I have a world where Hemisphere A is uninhabitable due to extreme heat(possibly due to coming close to its sun during its orbit) whilst Hemisphere B has pleasant Earth like conditions(temperature from 0-100F). As it orbits,it rotates so Hemisphere A becomes pleasant and Earth-like whilst Hemisphere B becomes uninhabitable due to extreme cold.

How feasibly could a planet's orbit create this?

• you were fine right up until "And what would be the effect on a population..." I suggest you remove this or your question will be closed as too broad.
– John
Commented May 22, 2018 at 15:02
• So within a normal planetary rotation, you effectively have 3 hemispheres, one extremely uninhabitably hot, one pleasant and one extremely uninhabitably cold? Is that really what you mean/want? Commented May 22, 2018 at 15:06
• A planet like this seems to be the setting for Harry Harrison's Wheelworld, volume II of his To the Stars! trilogy. Commented May 22, 2018 at 17:24
• So is the inhabitable hemisphere at any given time always dark? Commented May 22, 2018 at 17:58

## 5 Answers

Maybe this could be done with an extremely elliptical orbit. https://en.wikipedia.org/wiki/Elliptic_orbit

Here is how I envision it.

In this planet, the day is equal to the year - the planet turns on its axis once for every trip around its star. A is always day. B is always night.

• When A is hot the planet is close to the star, and A faces the star. It is night time on B and it is warm because of the heat delivered to A traveling over by convection.

• When it is far from the star A does not get as hot and so it is nice there. But the night side gets very cold.

• That was my immediate thought too. A tidally locked planet in a highly elliptical orbit. The main challenge I see is that B will probably still be too cold when it's getting no sun at all, so you will need a robust atmosphere to help distribute heat from the A side.
– Gene
Commented May 22, 2018 at 19:39
• "night time on B and it is warm" - Warm must be a relative term, like Antarctica is warm compared to the nighttime side of the Moon. Commented May 22, 2018 at 21:12

The orbit is simple it's one with no sidereal rotation, it's also impossible because stellar gravity will supply sidereal rotational energy at least sufficient to create tidal lock, this will be debited from the planet's orbital momentum and degrade said orbit rapidly.

No idea what it would do to a population because like I said it can't happen, not for long anyway.

• @Futoque Actually it could be a spin-resonant orbit of some kind, that might get closer to approximating the OP's asked for effects, my answer is basically a continuous sweep from warm to cold to warm, rinse and repeat for any given point as every orbit progresses.
– Ash
Commented May 22, 2018 at 15:50
• I removed the original comment because I had an assumption in there that was not right, and you beat me to the correction. The original comment was about Mercury's 3:2 spin orbit resonance and high eccentricity. The side facing the sun at aphelion would have a lower temperature than at perihelion. Commented May 22, 2018 at 15:53
• Uranus seems to be rotating on its side just fine for the last few billion years. Commented May 22, 2018 at 17:25
• @Alexander We have no idea how long its been doing that, we only found it a couple of hundred years ago, 1781 specifically, also it's so far out of the habitable zone that solar gravity input is pretty minimal, we don't know how fast Uranus's rotational axis is shifting, but it will be.
– Ash
Commented May 22, 2018 at 17:42
• A planet with one or more significantly massive moons will experience local tides that completely overwhelm any solar attraction. Such a planet could easily sustain a rotation on the order of, say, nine Earth years, for several hundred million years. Commented May 22, 2018 at 20:04

So I have a world where Hemisphere A is uninhabitable due to extreme heat

whilst Hemisphere B has pleasant Earth like conditions(temperature from 0-100F).

If your planet has an atmosphere this will not happen like this. The atmosphere will tend to move heat from the cold region to the hot one and the larger the temperature difference the stronger the winds.

As a result your planet's temperate ("nice") side will actually get quite a lot of heat and complex weather which will make it just as unpleasant.

(possibly due to coming close to its sun during its orbit)

You have a misconception about this.

There's a very approximate quantity known as the effective temperature which is a rough estimate of planetary temperature (ignoring atmosphere and rotation effects) which tells us that :

$$T_{eff}\propto \frac 1 {\sqrt R}$$

where $$T_{eff}$$ is temperature and $$R$$ is distance from the star.

Now $$R$$ is very large compared to the size of a planet ( if you want an inhabited planet ) and this means the difference between $$R$$ at the near side and $$R$$ at the far side is minute. You won't gain any temperature difference like this, it's too small an effect.

As it orbits,it rotates so Hemisphere A becomes pleasant and Earth-like whilst Hemisphere B becomes uninhabitable due to extreme cold.

Well heat won't just change instantly. You've heated up one side and as it rotates it distributes the heat via the atmosphere and also stores heat in the ground. Simply rotating won't change anything instantly as e.g. Earth's day and night show.

Now the vegetation, animals and insects and the water or liquids in the hot atmosphere won't just suddenly become part of a pleasant temperate climate. Instead the atmosphere will continue to act like a greenhouse (a bit like Venus maybe) and you'll still have a pressure cooker.

Let's look at Venus.

The effective temperature (our rough theory estimate) says Venus should be about 312 K compared to a theory value of about 275 K for Earth.

The actual values are 735 K for Venus and about 288 K for Earth.

The huge difference for Venus is because of it's atmosphere. Get too hot and your atmosphere becomes a runaway Greenhouse atmosphere.

There is practically no difference between day and night temperatures on Venus.

This is the direction your planet would head for - very quickly. Once you start putting a lot of water vapor into the atmosphere you have this problem.

How feasibly could a planet's orbit create this?

Impossible under any circumstances with an atmosphere.

Consider a binary planet situation. The pair orbits around the sun just like Earth does, and the pair spins around a point between them such that, on each planet, one side gets sunlight and night and the other side is permanent night because it is shaded by the other planet. Over time, the planet also turns on its axis, changing which side is in shadow. Put the pair close enough to the star so that the shadow side is the habitable side.

Tidal locking may slow your rotations — I’m not able to model this system... it may not be stable. But if I were modeling, something like this is where I’d start.

If we say that the orbit is possible, here's how the planet's general atmospheric circulation might look (first looking from the side, then from the sun's perspective):

Here, side A is shown along with the circulation and phenomena one might expect to find at different latitudes.